Hollow cathode plasma sources are commonly used in the art for coating and surface treatment applications. These plasma sources comprise one or more hollow cathodes electrically connected to a source of power. Several different types of hollow cathodes may be used in these plasma sources, including point sources or linear hollow cathodes.
Power sources used in hollow cathode plasma sources are typically configured to supply one of direct current, alternating current, or pulsed current (i.e., current having a square or rectangular waveform where the duty cycle is less than 100%) to the hollow cathodes. Bipolar power sources (i.e. two phase power supplies) are currently used to provide alternating or pulsed current to a hollow cathode plasma source.
The use of direct current during operation of a linear hollow cathode plasma source causes plasma to be generated primarily in a single area, rather than throughout the entire length of the linear hollow cathode. Although some types of plasma sources using direct current can effectively utilize magnets to generate a uniform plasma, this cannot be done with linear hollow cathodes. However, a high degree of uniformity (not achievable by the use of direct current in a linear hollow cathode plasma source) is necessary for many applications, such as coating glass using plasma-enhanced chemical vapor deposition.
The inventors previously discovered that, in a hollow cathode plasma source, using two-phase (bipolar) alternating or pulsed power can achieve a uniform linear plasma. However, the use of two-phase power in hollow cathode plasma sources has some disadvantages. For example, due to the alternation of the power, a plasma is not actively generated (i.e., there is no active electron emission) by the plasma source for some portion of the operational time. For typical applications, this time where plasma is not being actively generated is approximately 25% of a period of the power supply. Another disadvantage is that there is significant wear on the plasma source due to the use of the two-phase power source, which decreases the operational life of the plasma source.
Thus, there is a need in the art for plasma sources that overcome these and other disadvantages of known plasma sources.